1. Field of the Invention
The present invention relates, in general, to a method for fabricating a half-tone type phase shift mask, capable of preventing the penetration of light at unintended areas, and thus capable of obtaining a photosensitive pattern superior in smooth profile.
2. Description of the Prior Art
High integration in semiconductor devices has generally required that wires be reduced in size and that the distance between wires and unit devices, such as transistors and capacitors, become smaller. As a result, the number of steps involved in fabricating a highly integrated semiconductor are increased.
A typical photolithography process for semiconductor devices employs a mask comprising a transparent substrate, such as quartz, on which light screen patterns are formed. However, it is difficult to form a fine pattern smaller than the light resolution by using a typical mask. Indeed, it is virtually impossible to obtain fine patterns with a line width of 0.5 .mu.m or less by using current photosensitive solutions and light exposing equipments.
Meanwhile, very highly integrated semiconductor devices, such as 64 M or more DRAM, demand fine patterns of 0.4 .mu.m or less. Various efforts have been made to meet such demands. In an effort to develop highly integrated semiconductor devices, a phase shift mask was developed.
A phase shift mask is comprised broadly of a quartz substrate, light screen patterns and a phase shift material layer. The phase shift material layer is formed along with the light screen patterns on the quartz substrate, playing a role of shifting a beam of light at an angle of 180.degree.. Such phase shift mask is designed to keep constant the amplitude of a beam of light illuminated on a wafer in light exposure processes and to minimize the exposure caused by interference between a light passing through the phase shift material layer and a light passing through the adjacent quartz substrate, thereby improving the resolution of the photosensitive film pattern.
Of phase shift masks, a half-tone type phase shift mask is utilized to form contact hole patterns, and its process capability is proved to be better by 50% or more than other conventional phase shift masks. In processes for fabricating a semiconductor device, the half-tone type phase shift mask is especially useful in forming fine contact holes of 64 M or more scale.
In order to better understand the background of the present invention, a conventional half-tone type phase shift mask will be described in connection with FIGS. 1 and 2.
First, referring to FIG. 1, there is shown a cross section of a conventional phase shift mask. As shown in this figure, the conventional phase shift mask comprises a quartz substrate 11 on which a light screen pattern 13 along with a phase shift film pattern 15 are formed so as to provide a window 12 exposing the quartz substrate 11 therethrough. The light screen patterns 13 are made of chrome or chrome oxide and have such a thickness so as to permit penetration of about 5 to 10% of an incident light. The phase shift film pattern 15 is made of an oxide, nitride or SOG with a predetermined thickness.
When a light is illuminated on such a half-tone type phase shift mask, the intensity of light is maximized at the central area of the window 12 and reduced to zero at the edge area of the window, as shown in FIG. 2. This drawing also indicates that the intensity of light at areas provided with no window is, although very weak, detected. This is attributed to the fact that the light penetrates through the light screen pattern 13. Of course, the intensity of the passing-through light is proportional to that of a light source. In addition, the intensity of light is not constant but different at the areas in which a window is not provided, as shown in FIG. 2.
When a positive photosensitive film coated on a semiconductor substrate is illuminated by use of such a conventional half-tone type phase shift mask, the area which is intended not to be patterned becomes illuminated because of the penetration of light through the light screen and thus, patterned after development. As a result, a photosensitive film pattern having an uneven surface is obtained. This uneven surface is due to the fact that the incident light passes through the light screen into the unintended areas and the intensity of the passing light is different according to the unintended areas. That is to say, the photosensitive film pattern is formed at the desired height at the edge area by virtue of no intensity of light, but since the photosensitive film of the unintended areas is illuminated by weak and variable intensities of the passing light, the photosensitive film is removed at different thicknesses according to areas when developing.
Such a photosensitive film pattern with an uneven surface is a factor which decreases the production yield and reliability of a semiconductor device. For example, when the conventional photosensitive film pattern is used as an etching mask or an ion implantation mask, a layer under the mask may be damaged or ions may be implanted into unnecessary areas.